In magnetic particle inspection systems, a part is magnetized prior to or during inspection by passing a current through the part or by passing a current through a coil which surrounds the part, magnetic particles being applied over a surface portion of the part to be concentrated at regions where there are cracks or similar defects. Rectifiers have been generally used to develop a unidirectional magnetizing current from an AC supply, usually with a step-down transformer for developing a high current for passing through the part. Various types of controls have been provided including variable or tapped transformers. More recently, systems using silicon-controlled rectifiers (SCR's) have been used.
In one type of system using SCR's and operative from a single phase supply, a pair of oppositely poled SCR's in series between the single phase supply and a step-down transformer are controlled by controlling the phase angle of firing in relation to a reference phase point or points of the single phase supply. The reference phase points may be established by detecting the cross-over points of the single phase voltage. The phase control circuit, in turn, is controlled from a control signal developed in response to a current signal which is proportional to the magnetizing current and a reference signal level which is selected in order to select the output current. The control signal may also be responsive to a voltage signal proportional to the voltage applied to the part, the voltage signal being used to limit the applied voltage until the current through the part reaches a certain level. This type of control has been found to be advantageous in the system designed for single phase operation and for developing relatively low magnetizing currents for the testing of small parts.
For systems for testing of larger parts, three phase supplies have been used and in one type of system, adjustable potentiometers have been used to control the firing angles of SCR's connected in series with the primary windings of a three phase transformer, additional SCR's being provided in the secondary circuit for rectification and polarity control.